Polymer blends comprising a second polymer dispersed in a matrix of a first polymer are very useful and, depending on the properties and the relative amounts of the first and second polymers, a wide variety of such polymer blends can be produced. Of particular interest are polymer blends, also referred to as thermoplastic elastomers (TPE), in which the first polymer is a thermoplastic material, such as polypropylene, and the second polymer is an elastomeric material, such as an ethylene-propylene elastomer. Examples of such thermoplastic elastomers include polypropylene impact copolymers, thermoplastic olefins and thermoplastic vulcanizates.
Unlike conventional vulcanized rubbers, thermoplastic elastomers can be processed and recycled like thermoplastic materials, yet have properties and performance similar to that of vulcanized rubber at service temperatures. One method of making the aforementioned polymer blends is by melt mixing the two different polymers after they have been polymerized to achieve a target set of properties. However, this method is relatively expensive making it much more desirable to make blends by direct polymerization. Blending by direct polymerization typically uses multiple reactors in series, where the product from one reactor is fed to a second reactor having a different polymerizing environment, resulting in a final product that is an intimate mix of two different products. Examples of such processes employing vanadium catalysts in series reactor operation to produce different types of EPDM compositions are disclosed in U.S. Pat. Nos. 3,629,212, 4,016,342, and 4,306,041.
Currently, isotactic polypropylene is widely produced by a slurry polymerization process, whereas ethylene propylene copolymers having the desired properties for use in TPE applications are commercially produced in a solution process. However, existing solution processes are unable to produce polypropylene with high molecular weight and high melting point, both of which are important properties for TPE applications. Similarly, ethylene propylene copolymer elastomers have been difficult to produce using slurry-based polymerization systems since, even at low reactor temperatures, these tend to result in reactor fouling and the formation of rubbery clumps that attach themselves to the reactor agitator, necessitating frequent reactor shutdown. It is therefore difficult to produce a TPE material using either a slurry process (effective for the polypropylene component) or a solution process (effective for the rubber component).
Thus current in-reactor processes for producing propylene impact copolymers typically involve polymerizing propylene in a first reactor and transferring the polypropylene homopolymer from the first reactor into a secondary reactor where copolymerization of propylene and ethylene proceeds by virtue of still active catalyst on the polypropylene granules produced in the first reactor. No additional catalyst is fed into the secondary reactor, which is typically operated in a gas-phase process. However, only a limited amount of rubber phase can be produced in a gas phase process. Moreover, the rubbery phase material inherently has low melting or softening temperature. Under such conditions in either a fluidized or stirred gas-solid phase reactor, stickiness of the olefin polymer particles or granules becomes a problem. Ethylene copolymers using propylene, butene-1, and higher alpha comonomers are prone to stickiness problems when their crystallinity is below 30% or densities less than about 910 kg/m3. The stickiness problem becomes even more critical with copolymers of ethylene and propylene having a crystalline content less than 10%. Only limited amount of the low crystallinity copolymer can be produced in the gas-phase reactor due to the sticky nature of low crystallinity copolymer. Presence of the low crystallinity copolymer also limits the polymerization temperature to a relatively low level. Low process temperatures are generally undesirable due to reduced conversion efficiency and consequent increased costs of operation.
In addition to the issue of reaction medium, choice of polymerization catalyst is another factor that currently limits in-reactor blending of TPE materials. Thus, although polypropylene and ethylene propylene elastomers are produced commercially by both metallocene catalyst systems and Ziegler-Natta catalysts, polypropylene produced using a Ziegler-Natta catalyst and an elastomer produced using a metallocene catalyst are preferred in many applications. However, there is a general perception in the industry that Ziegler-Natta catalysts and metallocene catalysts are incompatible and hence there is currently no in-reactor process for producing blends of Ziegler-Natta-produced polypropylene with metallocene-produced elastomer.
According to the present invention, a hybrid process for producing polymer blends has now been developed in which a first monomer, such as propylene, is polymerized in a first slurry phase reaction zone in the presence of a supported first catalyst, which can be either a Ziegler-Natta catalyst or a metallocene catalyst, under conditions to produce a thermoplastic first polymer. At least part of the first polymer is then contacted with at least one second monomer, such as ethylene, in a second solution phase reaction zone in the presence of a second catalyst, which again can be either a Ziegler-Natta catalyst or a metallocene catalyst, under conditions to produce said second polymer.
U.S. Pat. No. 6,245,856 discloses a thermoplastic olefin composition comprising polypropylene, an ethylene-alpha olefin elastomer and a compatabilizer comprising an ethylene-propylene copolymer having a propylene content of greater than 80 weight percent. According to this patent, the individual components of the composition can be separately manufactured and mechanically blended together in a mechanical mixer or two or more of the components can be prepared as a reactor blend using a series of reactors where each component is prepared in a separate reactor and the reactant is then transferred to another reactor where a second component is prepared.
U.S. Pat. No. 6,207,756 describes a process for producing a blend of a continuous phase of a semi-crystalline plastic, such as polypropylene, and a discontinuous phase of an amorphous elastomer, such as a terpolymer of ethylene, a C3-C20 alpha olefin and a non-conjugated diene. The blends are produced in series solution polymerization reactors. U.S. Pat. No. 6,319,998 also discloses using series solution polymerizations to produce blends of ethylene copolymers. U.S. Pat. No. 6,770,714 discloses the use of parallel polymerizations to produce different polymeric components that are then blended through extrusion or using other conventional mixing equipment. One polymeric component is a propylene homopolymer or copolymer and the second polymeric component is an ethylene copolymer.
U.S. Pat. No. 6,492,473 describes a mixed transition metal olefin polymerization catalyst system suitable for the polymerization of olefin monomers comprising at least one late transition metal catalyst system and at least one different catalyst system selected from the group consisting of late transition metal catalyst systems, transition metal metallocene catalyst systems or Ziegler-Natta catalyst systems. Preferred embodiments include at least one late transition metal catalyst system comprising a Group 9, 10, or 11 metal complex stabilized by a bidentate ligand structure and at least one transition metal metallocene catalyst system comprising a Group 4 metal complex stabilized by at least one ancillary cyclopentadienyl ligand. The polymerization process for olefin monomers comprises contacting one or more olefins with these catalyst systems under polymerization conditions.